Electroless nickel/gold (Ni/Au) under bump metal (UBM) is a commonly used UBM for wafer bumping used in Flip Chip applications because it's a low cost maskless process. This process involves the dipping of silicon nitride (Si3N4)/polyimidecover/benzocyclobutene (BCB) passivated wafers into a series of plating baths to grow a thick layer of Ni studs (UBM). After this UBM process, the wafer is sent for bumping and reflow to form round solder ball interconnects.
However, the adhesion of this electroless Ni/Au UBM to the passivation layer is poor, and during plating, the plated Au will follow a capillary action and become deposited on the side wall of the PI/BCB passivation layer. During solder bump reflow, this will cause solder to flow into the side wall of the passivation layer as Au is proven to be a good wetting layer of solder. Thus, the solder will attack the underlying aluminum (Al) pad causing corrosion to the Al pad and affecting the reliability of the package.
U.S. Pat. No. 6,452,270 B1 to Huang describes increased adhesion between the UBM and passivation by using a Ti UBM with a closed-loop shape.
U.S. Pat. No. 6,521,996 B1 to Seshan describes a lower adhesion layer that provides good adhesion to the passivation layer.
U.S. Pat. No. 6,426,281 B1 to Lin et al. describes a passivation layer that increases the adhesion to the UBM.